GIS Project: Mapping the Long Island Sound Bridge

Developer: John Daly
Course: Mapping Sciences 710
Instructor: Prof Albrecht

The notion of a bridge spanning Long Island Sound has been discussed for a few decades now. Mostly it falls into 'what if?' or Extreme Engineering topics of conversation, but the ever-expanding city metropolitan population, and the congestion and traffic snarls already on city bridges, roads and tunnels make this a relevant topic today. It may even become a necessity in the near future. Since the placing of such a bridge is dependent upon population growth, ecological concerns and the location of major roads, this project is a very good subject for a GIS model.

There are seven counties surrounding Long Island Sound: in New York they are Suffolk, Nassau and Westchester. In Connecticut: Fairfield, New Haven, Middlesex and New London. The population growth per square mile of all of these counties will be broken down by census tract, the metric being the growth of each tract between the 1990 and 2000 Decennial censuses.

Critical environmental areas and sensitive tidal wetlands along coastal areas will be analyzed to rule out development in these locations. A Benthic survey has also been included.

Lastly, major roads (parkways, expressways, turnpikes and interstates) will be shown for each county. Visual inspection of these roads may determine a better bridge placement should more than one candidate appear.

Phase 1:

Acquire Census Data: Thematic data that includes population per square mile was downloaded from American Fact Finder. This information was obtained from the 1990 and 2000 censuses and collected for all seven counties.

Link: American Factfinder

Phase 2:

Normalize Census Data:Owing to differences in the format between the two Decennial censuses, it was necessary to ignore a small amount of data where tract ids had no ccrresponding records. Using unix editing tools the data files were then edited to create a one-to-one relationship of tract ids between censuses.

Link: Data Normalization

Phase 3:

Determine Spatial Extent and Find Entire Bounding Area: Even though the study focuses on the counties surrounding the Long Island Sound and the estuary itself, researchers have determined the complete study area is much larger, and therefore a shapefile was chosen with state boundaries appropriate for the project and its possible expansion. The Coordinate System and spatial extent imported with this Feature Class was accepted for the project:

Geographic coordinate system name: GCS_North_American_1983
Horizontal Datum Name: North American Datum of 1983

Bounding coordinates
Horizontal
In decimal degrees
West: -77.877236
East: -67.158958
North: 47.459687
South: 36.543782
In projected or local coordinates
Left: -77.877236
Right: -67.158958
Top: 47.459687
Bottom: 36.543782

Link: Project Boundary

Phase 4:

Import TIGER/Line shapefiles for each county: TIGER files were added to the project and were JOINed with the intermediate data files. A float field named GrowthRatio was then added so a future calculation could be made between the two PPSQM fields.

Phase 5:

Import Roads and Urban Areas: TIGER/Line shapefiles for Roads and Urban areas are imported into the project. Following best practices for database modeling, shapefiles with the same spatial extent were grouped together in a Feature Dataset. In this project each county is a Feature Dataset, including feature classes of Census Tract, Major Roads and Urban Areas

Link: Feature Datasets

Phase 6:

Ecological Concerns: Bathymetry and Benthic data were downloaded and imported from links on the USGS Science Center at Woods Hole site. This data has a Projected Coordinate System and a False Easting. A new Projected Coordinate System named Bridge_Coordinate_System was created for this and future imports for LIS ecological studies.

Projected Coordinate System: Bridge_Coordinate_System
Projection: Transverse_Mercator
False_Easting: 500000.00000000
False_Northing: 0.00000000
Central_Meridian: -75.00000000
Scale_Factor: 0.99960000
Latitude_Of_Origin: 0.00000000
Linear Unit: Meter

Links: USGS Science Center for Coastal and Marine Geology

Phase 7:

Coastal Vulnerability Data: Data for tidal wetlands and critical shoreline environments was sought out for each county. This was the most difficult data to find and success was had at only one location: Westchester County. The initial format of the files was Data Interchange Format .e00. These files were then converted to coverages using Import71 from ArcCatalog, and eventually added to the map.

Phase 8:

Add Data to Map: Create Map and Add Data Sources.

Phase 9:

Determine and Display Growth Rates: The GrowthRatio fields for all counties was calculates as ((ppsqm1 - ppsqm2) / ppsqm2) * 100, and the percent value truncated to one decimal place. Graduated colors were used with 5 classes of Natural Breaks. Tracts with over a 25 percent growth rate between census were selected and labeled

Link: Growth Rate

Phase 10:

Define Major Roads: The major roads shapefiles were exhaustive in detail. To pick the most important roads from these features, a Selection by Location was used with a query to display only highways, parkways, turnpikes and interstates.

Link: Major Roads

Phase 11:

Narrow Candidate Areas: By visual inspection the choice of bridge location was narrowed down to Westchester and Nassau Counties. The reasons are threefold: 1) Nassau County has the shoreline with the most rapidly growing census tracts. 2) The location of benthic communities narrows considerably towards the westward funnel of the sound 3) It is the shortest span on which to build.

Phase 12:

Find Bridge Areas: To find locations to place the bridge, tidal wetland areas on would have to be avoided as much as possible. To do this the area with the least amount of wetlands is located, and the existing wetlands buffered to an extent of 500 feet. The order of operation to do this in ArcMap is as follows:

a) Create Model
b) Find IDs of identified wetlands for buffering
c) Add SELECT process to filter IDs
d) Add BUFFER process to buffer wetlands 500 feet

Since free or easily obtainable environmental data sets were not found for the north shore of Nassau County, a land projection into the Sound was chosen for the Nassau coastal terminus of the bridge.

Link: Wetlands Buffer

Phase 13:

Draw and annotate bridge mapping: Lines were drawn connecting the points and an Annotation group added with the latitude and longitude coordinates for the final placements.

Link: Bridge Placements

Phase 14:

Format Map: A template of LandscapeModernInset was chosen for the map. Layer transparencies were adjusted and the map annotated to illustrate the approximate coordinates of the bridge placements. An inset map showing the entire project boundary was added along with legends for both data frames. The map was then given descriptive text and labelled.

Link: Map Template